Quote from MS
At startup, Svchost.exe checks the services part of the registry to construct a list of services that it must load. Multiple instances of Svchost.exe can run at the same time. Each Svchost.exe session can contain a grouping of services. Therefore, separate services can run, depending on how and where Svchost.exe is started.
To view and or post a list of what is running within the file, read this provided by Computerhaven
Before you post a load list, we need a beter understanding of the following phrase(s)..
(A) This problem occurs upon starting the computer and slows everything down.
(B) From time to time the computer becomes very slow and Task Manager shows sychost.exe taking 90% to 100% CPU
We need a approximate time in seconds this activity lasts. Depending on how old of a computer, how much memory you running, and how many processes are listed in Task manager can dramatically influence this time value . Also, some services may require "update checks" via internet .. are you on dialup or broadband service.
Does this occur when you do certain activities, example "Printing" or other hardware devices loaded. If yes, how long. Again, we could be at a resource thresh hold or their may be a real problem.